The invention relates to fiber lasers and more particularly to method and apparatus for pumping a fiber laser.
In a fiber laser, neodymium, or another rare earth such as erbium, terbium or praseodymium, is doped into the core of an optical fiber to provide an active gain medium. Pumping radiation input into the fiber causes lasing action in the fiber. Pump light at 800 nm, e.g. from a laser diode, produces an emission at 1.08 microns in a Nd-doped core. Single mode fibers are preferred for fiber lasers in order to produce single mode output. However, singe mode fibers have very small core diameters, typically 5-10 microns. Single stripe laser diodes can be focused into a single mode fiber core, but single stripe laser diodes are presently limited to about 30 mw output and thus are not well suited for pumping the fiber laser because of their low energy. Multi-spatial mode phase locked diode arrays and multi-spatial mode extended emitter laser diodes are presently available with output powers in the range 100-1000 mW or higher and are desirable as pump sources. However these high power diode arrays and extended emitter diodes produce multi-spatial mode output which cannot be effectively coupled into a single mode fiber core. Because of their poor beam quality, only about 10% of the light can be focused into the single mode fiber. Multi-mode fibers are much larger than single mode fibers, with typical core diameters of about 50-100 microns, so it is much easier to couple pump radiation into the fiber. However, a multi-mode fiber must be pumped harder to make it lase, typically requiring argon lasers or dye lasers as pump sources. In addition, the poor beam quality of the multi-spatial mode output makes the multi-mode fiber far less suitable than the single mode fiber for use in a fiber laser.
Thus the problem that presently exists in the development of a compact, low cost, reliable and efficient fiber laser is that the ideal combination is a single mode fiber pumped by a high power coherent laser diode source, either phase locked laser diode arrays or extended emitter laser diodes, but it is not possible to effectively couple these sources into a single mode fiber. Instead it is presently possible to pump a single mode fiber with a low power single stripe laser diode, or to use the high power coherent laser diode sources to pump a multi-mode fiber; neither of these combinations are particularly desirable because of the lower power source in the first case or the poor beam quality output in the second case.
U.S. Pat. No. 3,808,549 to Maurer, issued Apr. 30, 1974, discloses an optical waveguide light source in which light from an incoherent source, a plurality of LED's, is input into an optical fiber having a core of laser material surrounded by a first transparent cladding layer with a second cladding layer on the first layer. The refractive index of the first layer is less than that of the core and greater than that of the second layer, so that light from the incoherent source is propagated longitudinally along the fiber and coherent light is generated in the core of laser material by absorption of incoherent pump light propagating through the fiber. The coherent output from this source is coupled into a (fiber) waveguide. Thus Maurer shows a fiber optic coherent light generator, but does not use a coherent pumping source, does not use a single mode fiber, and does not form a complete laser.